In CFB boilers, the evaporation of heated feed water, i.e., boiling takes place mostly by means of water tube panels in the outer walls of the boiler furnace. When increasing the efficiency of the boiler, the cross-sectional area of the furnace must be increased proportionally with the efficiency, to be able to combust the required amount of fuel with a flow speed of oxygenous fluidizing gas corresponding to the original flow speed. Since it is not advantageous to form the shape of the horizontal cross section of the boiler to be very oblong, nor to increase the height of the boiler too much, the total area of the evaporator surfaces formed by the outer walls of the furnace tends to remain too small in large boilers. For example, if oxygen-enriched air is used instead of air as fluidizing gas, the surface area of the furnace walls available for evaporator surfaces may decrease even more. The additional need for evaporator surface area may also increase when using low-ash fuel with a good heat value, for example, dry coal.
To ensure a sufficient evaporator surface area in large boilers, it has been suggested to have different parts of additional evaporator surfaces disposed in the furnace. U.S. Pat. Nos. 3,736,908 and 5,215,042 disclose the division of the furnace by longitudinal, transverse or cross-wise water tube walls extending from wall to wall, the lower part of which has an opening or openings enabling the flow of material. U.S. Pat. No. 5,678,497 suggests the increase of heat exchange surfaces in the furnace by dividing the furnace into two by a longitudinal partition having short transverse wall portions connected thereto. Despite the openings in the partitions, both of the above-mentioned embodiments have a risk of not having the flows of the solid material and the gas in balance between the different parts of the divided furnace, which may, for example, increase environmental emissions or even cause an oscillating operation in the whole boiler. U.S. Pat. No. 6,470,833 discloses an arrangement, in which the operation of the furnace of the CFB boiler is improved by forming additional evaporator surfaces to separate, closed evaporator cavities extending from the bottom to the ceiling of the furnace. The disadvantage with these evaporator cavities is that they decrease the bottom surface area available, and increase heat exchange surface area only relatively little.